Fukushima Daiichi Unit 1 power plant containment analysis using GOTHIC

Abstract

This paper is a part of Fukushima Technical Evaluation Project (EPRI, 2013a, 2014a, 2015) which investigates various aspects of the Fukushima Daiichi event using the GOTHIC1GOTHIC incorporates technology developed for the electric power industry under the sponsorship of EPRI, the Electric Power Research Institute.1 code. The analysis takes advantage of the capability of GOTHIC to model certain aspects of the system geometry and behavior in more detail than typically considered in containment performance analysis. GOTHIC is a general purpose thermal hydraulics code that is used extensively in the nuclear industry for system design support, licensing support and safety analysis. It has the capability to model 3-dimensional flow behavior including the effects of turbulence, diffusion and buoyancy (EPRI, 2014b). This allows GOTHIC to be used in cases where mixing effects and stratification are important.The analysis presented here considers the events at Fukushima Daiichi Unit 1 (1F1) following the tsunami and leading up to the time of the hydrogen detonation in the 1F1 Reactor Building. The 1F1 MAAP5 Baseline Scenario (EPRI, 2013b) is used to define the steam, hydrogen and carbon-monoxide source terms from the primary system and the core concrete interaction. The model incorporates three dimensional modeling of the drywell, wetwell and connecting vent system that can predict the 3-dimensional flow patterns and the temperature and gas distributions. The model also includes leakage to the surrounding reactor building and the wetwell vent to the stack.The 3D containment model includes models for the heat transfer from the steam and gas in the drywell vent system to the torus room, wetwell gas space and pool. Inclusion of vent heat transfer had a significant impact on the overall containment response for the 1F1 scenario, particularly during the steam and hydrogen release from the primary system following the postulated failure of reactor vessel. Condensation in the vent system reduced transfer of noncondensing gases to the wetwell resulting in lower containment pressure and higher gas concentrations in the drywell.